Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

manual: Added <docref> XML Tag to Doxygen

Introduced a new Doxygen XML tag '<docref>'. The intention of
this tag is to indicate a section of text in the Doxygen XML that
will contain a reference to another section in the Manual e.g.
"See \autoref<sec:x>". As other generation formats aren't aware of
other chapters/sections in the manual, the <docref> encapsulation
allows it to omit the text from the output. The Latex generator
has been modified to continue parsing the 'docref' contents.

manual: allow non sec autorefs

Previously anything in an autoref block was assumed to reference
a section, which isn't true. Change 'sec' to 'label' and move the
'sec' prefix into the label itself.

libsel4: fix licenses

- some were incorrectly marked GPL (libsel4 is BSD)
- update NICTA --> DATA61 etc
- fix tags D61 --> DATA61
- update year to 2017

manual: Allow label_prefix to be set in idl

libsel4: Updated api xml dtd file with doc tags

update xml format to replace config with condition

tools: Updated DTD for conditional invocation labels

libsel4: Updating xml specification for syscall stub changes

Release snapshot